As Aer Lingus shares traded near to €3 for the first time, Ryanair faced questions about the wisdom of its own strategy. The airline's largest investor - New York-based fund manager Gilder, Gagnon, Howe & Co - was reported to have grilled executives in the airline as to why they were seeking to enter the long-haul market less than a week after the company told investors it had no plans for such a move.

During an investors conference call, the Gilder representative also questioned the timing of the bid. The attitude of US investors has raised the issue of whether Ryanair told investors about its move beforehand.

Mike Powell, an analyst at Dresdner Kleinwort Wasserstein, said US investors appeared to be taken aback by the takeover approach. "American investors were sounding quite confused. They probably don't know who Aer Lingus are."

Ryanair shareholders could force the company to abandon its pursuit of Aer Lingus, he claimed.

"There are a lot of investors who have been with Ryanair for a long time and who worship the company," Mr Powell added. "They are very confused by this. I would not be surprised if the deal collapses when Ryanair shareholders say 'why are you doing this?"